Method of applying a protective coating to metal



United States Patent 2,991,191 METHOD OF APPLYING A PROTECTIVE COATINGT0 METAL Earle T. Montgomery, Arnold P. Welch, and .loseph L. Bitonte,Columbus, Ohio, assignors to The Ohio State University ResearchFoundation, Columbus, Ohio, a corporation of Ohio No Drawing. Originalapplication May 10, 1949, Ser. No. 92,508, now Patent No. 2,775,531,dated Dec. 25, 1956. Divided and this application Aug. 31, 1956, Ser.No. 611,804

1 Claim. ('Cl. 117-22) The invention disclosed in this applicationrelates in general to coatings for metals, and more particularly toprocesses for forming such coatings and for adhering such coatings tothe metals to be coated; to articles comprising metals provided withsuch coatings; and to compositions of matter useful as such metalcoatings. The embodiments of our invention disclosed in illustrationthereof are particularly useful as coatings for withstanding hightemperatures and protecting the coated metals at these temperatures; andthe methods disclosed in illustration of our invention are particularlyuseful in forming such coatings and compositions, and in adhering suchcoatings and compositions to metals.

This application is a division of application Serial No. 92,508, filedMay 10, 1949, and now Patent No. 2,775,531.

One of the objects of our invention is the provision of a new coatingfor metals which is not only resistant to high temperatures but alsoresistant to oxidation and is at the same time a thermal insulatingcoating.

A further object of our invention is the development of processes forforming such coatings and for adhering such coatings to the metal to becoated.

A further object of our invention is the provision of an articlecomprising a metal, having alloyed or sinterbonded thereto, both arefractory and a heat and corrosion protective coating consisting of asinter-bonded mixture of a metal or alloy and a ceramic component whichmay be, for example, a metal oxide.

Another object of our invention is the production of a new material orcomposition of matter with a combination of properties making itsuperior as a protective coating for metal (especially at hightemperatures) to all coatings of the prior art, including the glass-typeenamels.

Further objects and features of our invention will be apparent from thereading of the following specification and claim.

We have designated our compositions of matter as cermets inasmuch asthey consist of a bonded mixture of extremely small particles of aceramic material, such as a metal oxide, and small particles of a metalor metal alloy. v

The compositions of matter, useful as coatings, with which we areprimarily concerned herein are therefore bonded mixtures of smallparticles of ceramic materials and small particles of metals or alloys.One example of such a cermet coating is a composition consisting of amixture of powdered nickel metal and powdered magnesium oxide(magnesia). The magnesia is preferably the electrically fused varietywhich has the greatest density and therefore a minimum shrinkage. Themixture of nickel and magnesia can be pressed into compacts orbriquettes and sintered to agglomerate and sinter-bond the components bya wetting reaction and solid solution between the metal and the ceramiccomponent through the media of 'a' limited oxide film formed on thenickel particles. Then the composition can be further prepared bycrushing the compacts and screening the powder preparatory toflame-spraying. The coating composition is then ready for directapplication to parts formed of sheet-metal, or.

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2 other metal parts, by flame-spraying with an oxyacetylene powder gun.

Alternatively, the raw and unsintered mixed powders alone; the sinteredand ground powder alone; or a mixture of raw powder and sintered powder,may be applied to the metal part to be coated, by a different process.For example, the coating in one of these forms, may be initially adheredby cold application using a fluid vehicle such as a solution of sodiumsilicate in water, by any of the coating methods known to the art andcommonly employed for the application of enamels, lacquers or paints.After drying the coating so initially applied, the coating may betorched with an oxyacetylene torch flame to agglomerate and. sinter-bondthe materials of which the coating is composed to each other and to themetal part, and preferably to alloy the metal or alloy component of thecoating to the metal part being coated. The same results may be obtainedby furnace sintering.

When the mild steels are to be coated by cold application of thecoating, as described above, which application is to be followed bytorching or furnace sintering, it has been found that the application ofan undercoat of flame sprayed or electroplated nickel, or otheroxidation resistant metal, onto the mild steel promotes better adherencebecause it inhibits the development of an oxide film on the mild steelat the interface between the steel and the coating, during the dryingand subsequent torch ing or furnace sintering operation.

We prefer very much that the surface to be coated should be sand-blastedbefore applying the coating.

7 Of the many metals which are suitable for use in our process, weprefer nickel, cobalt, aluminum, and iron, or alloys of these metals. Bylisting only four metals we do not limit ourselves thereto. By the termalloys of these metals we do not intend to limit ourselves to interalloys of the four metals mentioned, but we include alloys of any ofthese four or any other suitable metals with any of the others or withany suitable metal such as, for example, with chromium, columbium,tantalum, titanium, tungsten or zirconium. By metals we include anysuitable metals or any suitable alloy of such metals and we so definethe words metal and metals as used hereinafter in this specification andin the claim. Of the many metal oxides We prefer alumina, beryllia,magnesia, silica, and zirconia. We do not, however, wish to limitourselves to the five oxides mentioned. We have tested more thoroughlythan other coatings, those consisting of mixtures of magnesia andnickel. Of course, metals (e.g., chromium) other than those listedabove, are also useful, an oxides and even other similar compounds(e.g., silicon carbide) other than the ones listed may also be useful asceramics in our process of preparing coatings as also may be mixtures ofsuitable oxides. However, We prefer zircom'a or magnesia as a ceramiccomponent. The degree of usefulness of a given coating varies with themetal and ceramic component selected--and with the metal to be coated.The degree of usefulness varies in fact to a large extent depending onthe degree with which the metal and the ceramic component selected, weteach other and in turn wet or alloy with the metal to be coated. Theusefulness also varies with the temperature to which the coating issubjected in use. The greater the degree of wetting or agglomerationthere is, the greater is the usefulness of the combination, provided thecombination will also alloy with the metal to be coated, and issufficiently refractory for a given service.

We have found that when we sinter-bond a mixture of fine particles of ametal and a ceramic component to a metal sheet, for example, we canproduce a coating which will withstand much higher temperatures thanhave been previously possible with any coatings known in the prior artand even higher than the temperatures which can be withstood by themetal itself. At the same time the coating protects the sheet from.oxidation and corrosion; provides some thermal insulation, i.e.,reduces the density of heat transfer; and increases the normaltemperature differential between the heated and :unheated surfaces.

In sinter-bonding or alloying the coating to the metal to be coated, wereduce the particlesize of the presintered powder, and of both themetalcomponent and the ceramic component comprising the cold coatingmixture, to extremely small particles. The size of such particles is notcritical, but for flame spraying, the material should be less than 325mesh for the presintering operation and then after presintering, groundand screened to provide particles having a size of less than 100 mesh(149 microns) and greater than 325 mesh (44 microns), the screens usedbeing U.S. Standard brass sieves. The material for cold applicationshould be less than 325 mesh (44 microns and finer).

Then we adhere the mixture by flame-spraying or torching. Inflame-spraying the material passes through the flame. In heatingparticles of such size to the temperatures attained by them inflame-spraying, the metal particles bonded to the ceramic component ofthe presintered powder, reach, we believe, the plastic or tacky stage inthe oxyacetylene fiame, while in torching a mixed sintered andunsintered powder, it appears that in many cases a film of the oxide ofthe metal component forms on the surface of the metal or alloy componentand this metal oxide film goes into solid solution with the ceramiccomponent. In other cases we believe that there is a sinter-bondedagglomeration only, without wetting or solid solution. Although thetemperatures used are not necessarily critical for all mixtures, theoptimum temperature being dependent on the components, yet we preferusually to sinter at temperatures within ranges producing the conditionsabove referred to.

In bonding the particles to each other and to the metal to be coated, weuse a variety of methods. We may flame-spray the presintered and groundmixture of metal component and ceramic component particles onto thesurface to be coated. In so flame-spraying, we use a commericalPowder-Weld oxyacetylene powder gun. In the case of this oxyacetylenepowder gun or other similar types of high temperature torch guns, theflame may be surrounded with neutral nitrogen gas.

Alternatively, we may make an application of (.1) presintered and groundcoating alone, (2) a raw mixture of metal component and ceramiccomponent particles, or (3) a combination of (1) and (2), by using afluid vehicle, and spraying, brushing, dipping, or applying by any otherusual means, and after drying, either torch the coating with anoxyacetylene torch flame to sinter bond the materials of the coating andalloy or sinter-bond the coating to the metal part being coated, oraccomplish the same results by furnace sintering.

In preparing the mixture of fine metal or alloy and ceramic componentparticles, we may grind the materials separately then mix them togetherprior to presintering in the form of a compact, or prior to applyingthem raw and unsintered as a coating. For flame-spraying, which is ourpreferred method of application, we presinter the mixture. To do so wegrind the materials and mix them together and then form them into abriquette or compact by pressing (or otherwise) and sinter by heatingthe briquettes or compacts in a furnace, to a temperature which willagglomerate and/or sinter-bond the components. An oxidizing, neutral, orreducing atmosphere may be used in the furnace depending on thecomposition and on whether or not a passivated skin is formed on thecompacts. After sintering the compacts or briquettes, they are crushedto a fine powder of the order of less than 100 mesh (149 microns andfiner) and greater than 325 mesh (44 microns) and then applied as thecoating.

Direct flame-spraying of presintered powder, regardless of the characterof the components, is the preferred method of applying the coatingwherever the dimensions of the part to be coated will permit. In suchprocess the presintered and ground powder is sprayed through a torchflame as forexample an oxyacetylene torch, in such manner that theheated particles impinge on the surface being coated. For good adherenceof the flame-sprayed powder, the angle of incidence should be as near aspossible, although good adherence can be obtained with as low an angleas 45. There are often situations, however, where the dimensions of thepart to be coated on its inner surface will neither permit the entranceof the gun with the necessary clearance for flame-spraying, nor permitspraying the interior at an advantageous angle from a position outsidethe part. In such cases, either for the reasons just explained, forreasons of cost or practicability in building up thicker coats, or foradaptation to production line processing, we have recourse to the coldmethod of application of the coating, as described above, followed bytorching with an oxyacetylene torch flame or by furnace sintering.

So far as we know, we are the first to produce refractory, protective,thermal insulating, and hard surface coatings for metals, said coatingsconsisting of a mixture of -a metal or alloy powder (or flake) and aceramic or non-metal powder. Adherence of the coating to the metal partto be coated is achieved, we believe, by an alloying reaction betweenthe metal of the coating and the metal base. However, certain coatingcompositions will not alloy with certain metal bases and in such cases apurely mechanical adherence is achieved which is useful for applicationswhere the part is not subject to thermal cycling;

type of refractory coating has been developed particularly for servicein a temperature range above that in which the glass-type enamel groundcoats, and the glass-bonded refractory cover coats for enamels areuseful. The plastic stage of the most refractory high temperatureenamels is still below the plastic stage of the metals to which they canbe applied. Also, enamels which can be applied at temperatures even .ashigh as from 2300* to 2350 do not have an effective life even attemperatures of from 1800" F. to 2000 F.

The proportion of metal or alloy to ceramic material in the cermetcoating depends to some extent on the materials. However, .we prefer touse compositions within the range of a mixture having 25% metal and 75%ceramic material to a mixture having 75% metal and 25% ceramic material.One very satisfactory composition consists of 66.7% ground nickel and33.3% ground electrically fused magnesia (both percentages by weight).

Following are specific examples of the practice of various embodimentsof our inventions.

Example I Following is a description of our preferred method ofpreparing nickel-magnesia coating #10 for flame-spraying; and adescription of the spraying technique.

We obtained commercial electrically fused magnesia of the fineness of325 mesh and finer (approx. less than 44 microns) and commercial nickelmetal of the same fineness. We mixed together 200 grams (33.3%) magnesiaand 400 grams (66.7%) nickel, by dry grinding together for .30 minutesin a steel ball mill with steel balls. After mixing, the powder wasmoistened with 8% water containing a small amount of an organic binder(such as for example a starch suspension). It was then pressed intocompacts in a steel die, using approximately 12,000 p.s.i. After dryingat 250 F., the compacts were introduced into a Globar furnace withnormal oxidizing atmosphere, which had previously been brought to thetemperature of 2550" F. The compacts were sintered by soaking at thistemperature for 30 minutes. After cooling, the compacts were crushed,the material was .5 passed through a disc pulverizer, and screened in aRotap machine using U.S. Standard screens 100 mesh and 325 mesh. Thefraction which passed the 100 mesh screen but was retained on the 325mesh screen, was used for flame-spraying by means of a Powder-Weldoxyacetylene flame powder gun.

The metal surface to be flame-sprayed was cleaned and roughened by gritblasting using a sharp and fairly coarse silica sand or silicon-carbidegrit. The coating was applied by the same technique as used forflamespraying metal powders, which is well known to the art. The coatingis accomplished without undue heating of the target (the metal partbeing coated). The temper-ature attained by the target is controlled bythe distance from the gun to the target. Just enough heat is used toalloy a given coating to a given metal base.

Example II As described in the specification, a cermet coating may beinitially adhered to the metal part to be coated, by cold application inany one of three forms: (1) presintered and ground powder alone, (2) rawand unsintered mixed powders alone, or (3) a mixture of presinteredpowder and raw (unsintered) mixed powders.

Following is a description of our preferred method of preparingnickel-magnesia coating #10 for cold application, using a mixture ofpresintered powder and raw powder; and a description of the technique ofapplication. We obtained commercial electrically fused magnesia of thefineness of 325 mesh and finer (approx. less than 44 microns) andcommercial nickel metal of the same fineness. In addition we obtained bythe process described under Example I, presintered #10 Ni-MgO coatingmaterial of the fineness of 325 mesh and finer. We mixed together 100grams (16.7%) magnesia, 200 grams (33.3%) nickel and 300 grams (50.0%)presintered material of Example I, by dry grinding together for 30miuntes in a steel ball mill With steel balls. After mixing, the powderwas brought to the consistency of a slurry by stirring with a 30% sodiumsilicate solution and then brushed on the article to be coated, thesurface of which had been cleaned and roughened by sandblasting. Thecoating, so applied, was dried by infra-red radiation and finally in aconventional drier at 250 F. We then matured the coating throughagglomeration and sinter-bonding, and alloyed it to the metal part beingcoated by torching the coated surface with an oxyacetylene torch flamewithin the range 2100 F. to about 2500 F. Alternatively, this coating,as here described, has been initially applied by spraying instead ofbrushing, using a conventional glaze or enamel gun.

Example III Following is a description of the preparation of onecomposition of our aluminum alumina coating for flamespraying.

We obtained commercial calcined alumina reduced to particle size ofabout 44 microns and finer, and commercial aluminum flake reduced to athickness of about 5 milli-microns. We mixed together 150 grams (50%)alumina and 15 0 gnams (50%) aluminum in a porcelain mortar. Aftermixing, the material was dampened with water containing a small amountof organic binder and pressed into compacts, using approximately 1200p.s.i. These were dried slowly to 600 F. and sintered by placing in afurnace having a normal oxidizing atmosphere which had previously beenbrought to a temperature of 2500 F., and then soaking the compacts inthis furnace for 8 minutes to agglomerate and sinter-bond thecomponents. In sintering these compacts as here described, a tight andpassivated skin of aluminum oxide forms on all surfaces of the compact,leaving the metal content of the interior in the metallic state. Thecompacts were then crushed, ground, and screened as described underExample I. The fraction which passed the mesh screen but was retained onthe 325 mesh screen, was used for flame-spraying as described underExample I. 1

Example IV 7 Following is a description of our preferred method ofpreparing nickel-zirconia coating for flame-spraying.

We obtained commercial fused, stabilized zirconia of the fineness of 325mesh and finer (approx. less than 44 microns) and commercial nickelmetal of the same fineness. We mixed together 200 grams (33.3%) zirconiaand 400 grams (66.7%) nickel, by dry grinding together for 30 minutes ina steel ball mill with steel balls. After mixing, the powder wasmoistened with 8% water containing a small amount of temporary binder:It was then pressed into compacts in a steel die, using approximately12,000 p.s.i. After drying at 250 F., the compacts were sintered byplacing in a furnace having a normal oxidizing atmosphere and which hadpreviously been broughtto a temperature of 2550 F., and then soaking thecompacts in this furnace for 30 minutes to agglomerate and sinter-bondthe components. The compacts were then crushed, ground, and screened asdescribed under Example I. The portion which passed the 100 mesh screenbut was retained in the ,325 mesh screen, was used for flame-spraying asdescribed under Example I.

Example V Following is a description of our preferred method ofpreparing cobalt-magnesia coating for flame-spraying.

We obtained commercial electrically fused magnesia of the fineness of325 mesh and finer (approx. less than 44 microns) and commercial cobaltmetal of the same fineness. We mixed together 200 grams (33.3%) magnesiaand 400 grams (66.7%) cobalt, by dry grinding together for 30 minutes ina steel ball mill with steel balls. After mixing, the powder wasmoistened with 8% water containing a small amount of temporary binder.It was then pressed into compacts in a steel die, using ap proximately12,000 p.s.i. After drying at 250 F., the compacts were sintered byplacing in a furnace having a normal oxidizing atmosphere which hadpreviously been brought to a temperature of 2550 F., and then soakingthe compacts in this furnace for 20 minutes to agglomerate andsinter-bond the components. The compacts were then crushed, ground, andscreened as described under Example I. The portion which passed the 100mesh screen but was retained on the 325 mesh screen, was used forflame-spraying as described under Example I.

In addition mixtures of nickel and zirconia and mixtures of cobalt andmagnesia have each been used separately to form nickel-zirconia orcobalt-magnesia coatings by cold application in a manner analogous tothat described under Example II.

It is to be understood that the above described embodiments of ourinvention are for the purpose of illustration only and various changesmay be made without departing from the spirit and scope of ourinvention.

We claim:

A method of applying a protective coating to a metal article whichcomprises mixing nickel met-a1 particles of the fineness of 325 mesh andfiner and magnesia particles of the fineness of 325 mesh and finer inproportions of about 66%% by weight of particles of the metal and about33 /3 by weight of the particles of metal oxide; compacting theparticles; sintering in an oxidizing atmosphere and at an oxidizingtemperature; regrinding the sintered mixture to particles of less than325 mesh; applying said particles while relatively cold to the articleto be coated, and heating the article coated with said particles in afurnace at a temperature sufiiciently high to 7 sinter sa'id particlesto each other and adhere them to 2,161,597 the article to be coated.2,496,971

References Cited in the file of this patent UNITED STATES PATENTS 527,172 704,793 Griffith July 15, 1902 543,773 1,922,254 McCulloch Aug.'15, 1933 596,626

-8 Swartz June 6, 1939 Wiezer Feb. 7, 1950 FOREIGN PATENTS Great Britain1903 Great Britain Mar. 12, 1942 Great Britain Jan. 7, 1948

